1. Field of the Invention
The invention relates to a column unit for a chair having a fixed upright tube with a bottom upright-tube end and a top upright-tube end, a telescopic tube axially displaceably guided in the upright tube in the region of the top upright-tube end, a pneumatic spring having a cylinder and a piston rod, the cylinder being axially displaceably guided in the telescopic tube, the piston rod projecting out of the cylinder of the pneumatic spring and having a free end that is fastened on the upright tube proximate the bottom upright-tube end, the cylinder having a carry-along stop for carrying the telescopic tube out of the upright tube when the pneumatic spring is moved axially, and a securing element which limits an axial movement of the telescopic tube out of the upright tube.
2. Description of the Related Art
In known column units, a telescopic tube arranged in an axially movable manner in an upright tube is protected against undesired removal. For this purpose, the telescopic tube has a drawout-limiting pin which is subjected to loading by a leaf spring and engages a slot defined in the upright tube. The displacement which can be executed by the telescopic tube in relation to the upright tube is determined by the length of the slot. This method of securing against the telescopic tube being released from the upright tube requires high mechanical outlay. On the one hand, a slot has to be produced in the upright tube. On the other hand, it is necessary to install the drawout-limiting pin with the release spring in the telescopic tube.
Another known column unit includes a securing means with a drawout-limiting ring arranged in an annular recess of the telescopic tube. In this unit, a guide bushing is arranged in the upright tube having an internal diameter that is smaller than the external diameter of the drawout-limiting ring and larger than the external diameter of the telescopic tube. The bottom end of the guide bushing forms a stop against which the drawout-limiting ring strikes when the telescopic tube is extended. The arrangement of the drawout-limiting ring in an annular recess of the telescopic tube results in an increase in the overall length of the telescopic tube which cannot be utilized for the height adjustment. When a predetermined minimum drawout force is exceeded during withdrawal, the elasticity of the drawout-limiting ring allows it to be forced out of the annular recess by the stop and stripped off. A drawout-limiting ring with a high elasticity allows easy removal for maintenance. A disadvantage of this is that the drawout limiting ring has a low minimum drawout force, which may result in undesired removal and thus in the components failing completely. Added to this is the further disadvantage that the known securing means are arranged at the end of the telescopic tube. In the case of the column unit being subjected to force deviating from the longitudinal axis, the highest loading occurs in the region of the securing means, which reduces the service life of the securing means.